A top thread is generally of a great length by 3000 m to 4000 m and visible so that it rarely causes the problem of its running out. However, A bobbin thread is inevitably so short by 20 m to 50 m that it has to be changed quite often with another one, which ends up resulting in the unavoidable problem of uncertainty in determining when to change a run-out bobbin thread with a new one.
Herein lies the reason that a bobbin thread is inevitably short and should be frequently replaced: When a bobbin thread is stitched in lockstitch with a top thread with the help of rotation of a hook-set according to the working properties of a sewing machine, the size of a hook-set and the space for the bobbin thread to be placed are too limited to be enlarged infinitely.
In determining the timing of replacing a run-out bobbin thread with a new one, a sewing operator can reduce the problems caused by running out of a bobbin thread, by precisely recognizing its finishing point before its running out.
Failure of sensing the ending point of a bobbin thread causes a sewer to a loose stitch without a bobbin thread after its finishing up, and it thus results in defective products. In addition, as a connected sewing is followed with a second bobbin thread replaced after the finish up of a first bobbin thread, it results in the problem of the seam opening in high tension requiring products such as air bags, footwear, swimwear, and the like.
Most of the conventional apparatuses for solving such problems generally detect the ending point of a bobbin thread making use of an optical sensor. Such devices, however, have a drawback of low degree of accuracy in sensing the finishing point of a bobbin thread. That is because those detecting apparatuses using an optical sensor may be able to begin to transmit or reflect the light even when a bobbin thread is not finished up, but partially undone.